NEET MDS Lessons
Physiology
Reflexes
A reflex is a direct connection between stimulus and response, which does not require conscious thought. There are voluntary and involuntary reflexes.
The Stretch Reflex:
The stretch reflex in its simplest form involves only 2 neurons, and is therefore sometimes called a 2-neuron reflex. The two neurons are a sensory and a motor neuron. The sensory neuron is stimulated by stretch (extension) of a muscle. Stretch of a muscle normally happens when its antagonist contracts, or artificially when its tendon is stretched, as in the knee jerk reflex. Muscles contain receptors called muscle spindles. These receptors respond to the muscles's stretch. They send stimuli back to the spinal cord through a sensory neuron which connects directly to a motor neuron serving the same muscle. This causes the muscle to contract, reversing the stretch. The stretch reflex is important in helping to coordinate normal movements in which antagonistic muscles are contracted and relaxed in sequence, and in keeping the muscle from overstretching.
Since at the time of the muscle stretch its antagonist was contracting, in order to avoid damage it must be inhibited or tuned off in the reflex. So an additional connection through an interneuron sends an inhibitory pathway to the antagonist of the stretched muscle - this is called reciprocal inhibition.
The Deep Tendon Reflex:
Tendon receptors respond to the contraction of a muscle. Their function, like that of stretch reflexes, is the coordination of muscles and body movements. The deep tendon reflex involves sensory neurons, interneurons, and motor neurons. The response reverses the original stimulus therefore causing relaxation of the muscle stimulated. In order to facilitate that the reflex sends excitatory stimuli to the antagonists causing them to contract - reciprocal activation.
The stretch and tendon reflexes complement one another. When one muscle is stretching and stimulating the stretch reflex, its antagonist is contracting and stimulating the tendon reflex. The two reflexes cause the same responses thus enhancing one another.
The Crossed Extensor Reflex -
The crossed extensor reflex is just a withdrawal reflex on one side with the addition of inhibitory pathways needed to maintain balance and coordination. For example, you step on a nail with your right foot as you are walking along. This will initiate a withdrawal of your right leg. Since your quadriceps muscles, the extensors, were contracting to place your foot forward, they will now be inhibited and the flexors, the hamstrings will now be excited on your right leg. But in order to maintain your balance and not fall down your left leg, which was flexing, will now be extended to plant your left foot (e.g. crossed extensor). So on the left leg the flexor muscles which were contracting will be inhibited, and the extensor muscles will be excited
Conductivity :
Means ability of cardiac muscle to propagate electrical impulses through the entire heart ( from one part of the heart to another) by the excitatory -conductive system of the heart.
Excitatory conductive system of the heart involves:
1. Sinoatrial node ( SA node) : Here the initial impulses start and then conducted to the atria through the anterior inter-atrial pathway ( to the left atrium) , to the atrial muscle mass through the gap junction, and to the Atrioventricular node ( AV node ) through anterior, middle , and posterior inter-nodal pathways.
The average conductive velocity in the atria is 1m/s.
2- AV node : The electrical impulses can not be conducted directly from the atria to the ventricles , because of the fibrous skeleton , which is an electrical isolator , located between the atria and ventricles. So the only conductive way is the AV node . But there is a delay in the conduction occurs in the AV node .
This delay is due to:
- the smaller size of the nodal fiber.
- The less negative resting membrane potential
- fewer gap junctions.
There are three sites for delay:
- In the transitional fibers , that connect inter-nodal pathways with the AV node ( 0.03 ) .
- AV node itself ( 0.09 s) .
- In the penetrating portion of Bundle of Hiss ( 0.04 s) .
This delay actually allows atria to empty blood in ventricles during the cardiac cycle before the beginning of ventricular contraction , as it prevents the ventricles from the pathological high atrial rhythm.
The average velocity of conduction in the AV node is 0.02-0.05 m/s
3- Bundle of Hiss : A continuous with the AV node that passes to the ventricles through the inter-ventricular septum. It is subdivided into : Right and left bundle. The left bundle is also subdivided into two branches: anterior and posterior branches .
4- Purkinje`s fibers: large fibers with velocity of conduction 1.5-4 m/s.
the high velocity of these fibers is due to the abundant gap junctions , and to their nature as very large fibers as well.
The conduction from AV node is a one-way conduction . This prevents the re-entry of cardiac impulses from the ventricles to the atria.
Lastly: The conduction through the ventricular fibers has a velocity of 0.3-0.5 m/s.
Factors , affecting conductivity ( dromotropism) :
I. Positive dromotropic factors :
1. Sympathetic stimulation : it accelerates conduction and decrease AV delay .
2. Mild warming
3. mild hyperkalemia
4. mild ischemia
5. alkalosis
II. Negative dromotropic factors :
1. Parasympathetic stimulation
2. severe warming
3. cooling
4. Severe hyperkalemia
5. hypokalemia
6. Severe ischemia
7. acidosis
8. digitalis drugs.
The hypothalamus is a region of the brain. It secretes a number of hormones.
- Thyrotropin-releasing hormone (TRH)
- Gonadotropin-releasing hormone (GnRH)
- Growth hormone-releasing hormone (GHRH)
- Corticotropin-releasing hormone (CRH)
- Somatostatin
- Dopamine
All of these are released into the blood, travel immediately to the anterior lobe of the pituitary, where they exert their effects.
Two other hypothalamic hormones:
- Antidiuretic hormone (ADH) and
- Oxytocin
travel in neurons to the posterior lobe of the pituitary where they are released into the circulation.
Serum Lipids
|
LIPID |
Typical values (mg/dl) |
Desirable (mg/dl) |
|
Cholesterol (total) |
170–210 |
<200 |
|
LDL cholesterol |
60–140 |
<100 |
|
HDL cholesterol |
35–85 |
>40 |
|
Triglycerides |
40–160 |
<160 |
- Total cholesterol is the sum of
- HDL cholesterol
- LDL cholesterol and
- 20% of the triglyceride value
- Note that
- high LDL values are bad, but
- high HDL values are good.
- Using the various values, one can calculate a
cardiac risk ratio = total cholesterol divided by HDL cholesterol - A cardiac risk ratio greater than 7 is considered a warning.
Concentration versus diluting urine
Kidney is a major route for eliminating fluid from the body to accomplish water balance. Urine excretion is the last step in urine formation. Everyday both kidneys excrete about 1.5 liters of urine.
Depending on the hydrated status of the body, kidney either excretes concentrated urine ( if the plasma is hypertonic like in dehydrated status ) or diluted urine ( if the plasma is hypotonic) .
This occurs thankful to what is known as countercurrent multiplying system, which functions thankfully to establishing large vertical osmotic gradient .
To understand this system, lets review the following facts:
1. Descending limb of loop of Henle is avidly permeable to water.
2. Ascending limb of loop of Henly is permeable to electrolytes , but impermeable to water. So fluid will not folow electrolytes by osmosis.and thus Ascending limb creates hypertonic interstitium that will attract water from descending limb.
Pumping of electrolytes
3. So: There is a countercurrent flow produced by the close proximity of the two limbs.
Juxtamedullary nephrons have long loop of Henle that dips deep in the medulla , so the counter-current system is more obvious and the medullary interstitium is always hypertonic . In addition, peritubular capillaries in the medulla are straigh ( vasa recta) in which flow is rapid and rapidly reabsorb water maintaining hypertonic medullary interstitium.
In distal tubules water is diluted. If plasma is hypertonic, this will lead to release of ADH by hypothalamus, which will cause reabsorption of water in collecting tubules and thus excrete concentrated urine.
If plasma is hypotonic ADH will be inhibited and the diluted urine in distal tubules will be excreted as diluted urine.
Urea contributes to concentrating and diluting of urine as follows:
Urea is totally filtered and then 50% of filtrated urea will be reabsorbed to the interstitium, this will increase the osmolarity of medullary interstitium ( becomes hypertonic ). Those 50% will be secreted in ascending limb of loop of Henle back to tubular fluid to maintain osmolarity of tubular fluid. 55% of urea in distal nephron will be reabsorbed in collecting ducts back to the interstitium ( under the effect of ADH too) . This urea cycle additionally maintain hypertonic interstitium.
Bronchitis = Irreversible Bronchioconstriction
. Causes - Infection, Air polution, cigarette smoke
a. Primary Defect = Enlargement & Over Activity of Mucous Glands, Secretions very viscous
b. Hypertrophy & hyperplasia, Narrows & Blocks bronchi, Lumen of airway, significantly narrow
c. Impaired Clearance by mucocillary elevator
d. Microorganism retension in lower airways,Prone to Infectious Bronchitis, Pneumonia
e. Permanent Inflamatory Changes IN epithelium, Narrows walls, Symptoms, Excessive sputum, coughing
f. CAN CAUSE EMPHYSEMA
The Types of muscle cells. There are three types, red, white, and intermediate.
|
White Fibers Fast twitch Large diameter, used for speed and strength. Depends on the phosphagen system and on glycolysis-lactic acid. Stores glycogen for conversion to glucose. Fewer blood vessels. Little or no myoglobin. |
Red Fibers Slow twitch Small diameter, used for endurance. Depends on aerobic metabolism. Utilize fats as well as glucose. Little glycogen storage. Many blood vessels and much myoglobin give this muscle its reddish appearance. |
Intermediate Fibers: sometimes called "fast twitch red", these fibers have faster action but rely more on aerobic metabolism and have more endurance. Most muscles are mixtures of the different types. Muscle fiber types and their relative abundance cannot be varied by training, although there is some evidence that prior to maturation of the muscular system the emphasis on certain activities can influence their development